The present invention relates to a process and a device for representing the horizon on board an aircraft and, more particularly, a process and a device of this type which can be used in the flying aid system of the type known as "head up" which is found on board most modern aircraft.
It is known that the aircraft flying aid known as "head up" consists in projecting to infinity in front of the pilot's eyes symbols which can represent certain flight parameters of the aircraft in question, or even certain details of its environment. The pilot can thus know the information represented by various symbols, without having to consult the instrument panel of the aircraft or of its simulator, with the result that he can fly head up for most of the time. In addition, as the symbols are projected to infinity, the pilot has no need to make eye accommodation efforts in order to take note of the information that they represent.
The optical projection is therefore carried out by a device called a "head up collimator" which in particular includes a display device like, for example, a cathode ray tube, and optical devices enabling the projection of its screen to infinity to be carried out.
A "head up" flying aid system therefore includes several sensors of flight parameters connected to a processing and transmission chain which ends in a generator of symbols representing the parameters in question. It is this symbols generator itself which controls the collimator's display device which is most often a cathode ray tube.
More particularly as regards the representing of the horizon, the parameters whose values are detected by the sensors are the angles of pitch .theta., roll .phi. and heading .psi., as well as the roll and pitch velocities referenced p and q, respectively.
Instruments are known which permit the representation of the land horizon on board an aircraft with respect to the position of the aircraft. These devices usually include a sphere or a cylinder slaved to the angles of pitch, roll and heading. The two halves, in the case of a sphere the two hemispheres, are of different colors, the upper part generally being blue and the lower part maroon or black. Only the front surface of the sphere is visible, which permits, by appreciation of the quantity of blue or maroon seen by the observer, and by considering the orientation of the line of separation between the two hemispheres, the evaluation of the angles of pitch and roll of the aircraft. Graduated lines marked on the sphere also enable more accurate information to be obtained by reference to a fixed mark representing the axes of the aircraft.
The image which the observer receives from the sphere which forms the basic element of the traditional instrument described above is also known to be transposed onto a cathode ray tube. However, such a transposition cannot be carried out in the case of a "head up" collimator as the cathode ray tubes of such devices are monochromatic, such that, obviously, it is impossible to reproduce on their screen the blue and maroon or black areas formed by the sphere or cylinder of known devices. Moreover, an additional obstacle also comes from the fact that the image supplied by the cathode ray tube must not be overloaded in order that the cathode ray tube can present information other than the representation of the horizon and so that other symbols can be read through this representation of the horizon and so that the external terrain can also be seen.
In order to palliate these difficulties, it has also been proposed to represent the horizon on the cathode ray tube of a "head up" collimator by a line whose inclination with respect to the horizontal corresponds with the roll angle, while the pitch angle is represented by marks which become more separated from the line representing the horizon as this angle becomes larger, two different types of configuration given to these marks enabling the sign of the pitch angle to be distinguished, and thereby determining whether the aircraft is in the process of nosing down or of nosing up.
This solution is a last resort as the marks corresponding to the angles of pitch are hardly visible and there is a real risk of confusion between those of these marks which correspond to an aircraft in nose up and those which appear when this aircraft is in the process of nosing down, particularly when the horizon line is not visible or again in the case of a flight simulator where the pilot does not feel the physical sensation of the position of his aircraft. An additional disadvantage appears when the roll and pitch angles become high in value as it can arise that the marks in question purely and simply disappear from the screen of the "head up" collimator.